Computer networks that use the Internet protocol are commonly referred to as "IP networks." Within IP networks, host systems and other objects are identified by thirty-two bit numbers, known as Internet Protocol Addresses (IP addresses). IP addresses provide a simple mechanism for identifying the source and destination of messages sent within IP networks.
Increasingly, IP addresses within IP networks are assigned using the Dynamic Host Configuration Protocol (DHCP) defined in Internet RFC 1541 which is incorporated herein by reference. In networks that use the DHCP protocol, client systems request IP addresses from a DHCP server. The DHCP server allocates an IP address for use by the requesting client system and sends the client a message telling the client system which IP address to use.
The IP address allocated by the DHCP server is "leased" to the client system for a fixed period of time. Subsequently, the client system is responsible for periodically renewing the lease of the IP address. The client system and the DHCP server use an object known as a lease identification cookie to identify the client's lease. The client system may choose the lease identification cookie by sending the DHCP server a client identifier. Alternatively, the DHCP server uses the client system's IEEE-style MAC address as the lease identification cookie. The client system periodically renews its lease by sending a lease renewal message to the DHCP server that includes the lease identification cookie.
Unfortunately, allocation of IP addresses using traditional DHCP servers is subject to several possible attacks. One such attack is known commonly as "IP address hijacking." IP address hijacking occurs when a first client system acquires the IP address of a second client system. The first client maintains the hijacked IP address by sending lease renewal messages to the DHCP server using the second client system's lease identification cookie. In fact, since host names are commonly used as lease identification cookies, it is even possible for this type of attack to happen inadvertently. IP address hijacking confuses the network since a single IP address is now used by more than one host. As a result, network performance is degraded.
A second type of attack is known as "IP address hogging." For an attack of this type, a client system attempts to exhaust the supply of IP addresses by repeatedly obtaining IP leases from the DHCP server. Once the client system has leased all of the available IP address leases, network performance degrades as legitimate users are forced to wait for IP addresses.
Based on the foregoing, it may be appreciated that there is a need for DHCP servers that discourage both IP address hijacking and IP address hogging.